Evolution not revolution is the way for constitutions to develop - except, perhaps, at times in history when a real revolution is under way. That was the message of justice secretary Jack Straw yesterday, as he spoke to a Washington audience. The tumultuous circumstances of American independence allowed the founding fathers to build up enduring rules from first principles; in more ordinary times a constitution written on a blank sheet will provide only a paper barrier against the abuse of power.

Mr Straw is charged with fulfilling the prime minister's ambition of rationalising Britain's arcane system of governance. The title of yesterday's speech - modernising Magna Carta - reflected his gradualist pitch, and left him open to the charge of being more interesting in harking back to 1215 than driving forward towards a modern republic. But there is wisdom in the view that checks, balances and rights work best when the people have grown attached to them: only then will leaders fear the political price of overriding the rules. The real test for the government is not whether it can deliver a modern, codified constitution in the next few years, but whether it is moving towards that goal.

The past decade has seen important advances, from the human rights act to the overhauling of the traditional lord chancellorship, which stood in the way of the separation of judicial and executive power. Gordon Brown seemed to be stepping up the pace when, on becoming prime minister, he announced a clutch of worthwhile changes, including the requirement that parliament should always have a vote on the decision to go to war. But that reforming impulse has not been sustained, as was betrayed by last month's damp squib of a report on the electoral system - a missed opportunity to make voting fair. And Mr Straw's big idea yesterday for the next stage of reform is of dubious worth.

He is not alone in banging the drum for a "British bill of rights"; the opposition are doing so too. This bipartisan percussion sounds good, but the proposal is not what it seems, and many human rights groups oppose it. Shamefully, the Conservatives use the idea to create the impression they would replace foreign rights with British common sense, something achievable only by quitting Europe entirely. Mr Straw is up to nothing so sinister. He sees developing a British bill not as a way to roll back rights, but as a means to connect them with history and weave them more tightly into the culture. If the initiative spreads awareness of rights, it could prove its worth but it must not come at the cost of substantive modernisation. Public education should be a complement to real constitutional reform, not - as the government seems to believe - a substitute for it.